Many radiation detectors, such as a Focal Plane Array (FPA) of infra-red (IR) radiation detectors, output a current that must be amplified prior to signal processing. The Capacitive Transimpedance Amplifier (CTIA) is one conventional circuit that is often used for this purpose. By example, reference can be had to U.S. Pat. No. 4,786,831, entitled "Integrating Capacitively Coupled Transimpedance Amplifier", by Arthur L. Morse, Steven D. Gaalema, Ingrid M. Keimel, and Mary J. Hewitt, the disclosure of which is incorporated by reference herein in its entirety.
FIG. 1 illustrates a conventional CTIA configuration. A capacitor (C1) and a reset switch (SW) are connected in the feedback path of a high gain inverting amplifier (AMP) that provides charge buffering for an input current. The input current acts to debias the input of the CTIA. The amplifier drives the output node as a function of the input debias multiplied by the amplifier gain. This causes charge to be transferred from the input node to the capacitor C1.
In most such charge integration and charge amplification applications any noise that is present in the input charge signal is proportional to the square root of the input charge signal. The signal-to-noise ratio is therefore the least at the smallest input signal levels. However, linear charge integration and charge amplification circuits that are designed to accommodate the maximum input signal levels have difficulty in meeting the low noise requirements of the smallest input signals.